A peripheral nerve block (PNB) is used for anesthesia, postoperative analgesia, and diagnosis and treatment of chronic pain syndromes. Peripheral nerve blocks may also improve acute pain management and patient disposition even when used only as adjunct techniques. An objective of the PNB regional anesthesia technique is to identify the target nerve and position a hollow-bore needle in a defined proximity relative to the targeted nerve without causing untoward reactions such as structure damage to the nerve or causing excessive pain to the patient.
The current state-of-the-art for performing the PNB technique relies upon an adjunctive technology such as ultrasound to determine the general location of the needle relative to the position of the nerve and vascular components surrounding the nerve. Approximately 80% of PNB procedures performed utilize ultrasound as a primary means of locating the nerve. In fact, the medical literature currently advocates using ultrasound and notes that nerve stimulation is non-specific and unreliable.
Referring to FIG. 1, a description of the microanatomy of the peripheral nervous system is provided. The basic building block to both the central and peripheral nervous system is the single cell unit commonly known as is the axon. The brain and central nervous system are composed of millions of axons. Branching off the central nervous system of the brain stem and spinal cord is a collection of highly organized axons forming a network of sensory and motor pathways via the axons. This network of pathways is collectively known as the peripheral nervous system.
In the peripheral nervous system, each individual axon is surrounded by supporting connective tissue called the endoneurium. Contained within the endoneurium are small blood vessels (capillaries and venuoles) providing nutrients to these axons. Axons are collectively formed into highly organized, packed bundles that are surrounded by a thin but dense multi-layered connective tissue sheath that surrounds and forms a membrane structure called the perineurium. The perineurium provides a dense protective layer that is both a physical and chemical barrier, providing a degree of protection for the axons and endoneurium. This barrier is akin to the blood-brain barrier.
This discrete unit of the endoneurium and perineurium is called a peripheral nerve fascicle. When fascicles coalesce together they form fascicular bundles embedded in epineurium, which is a connective tissue sometimes referred to as inner epineurium. The multiple groups of fascicles are embedded in a non-uniform matrix of connective tissue (fibro-adipose tissue) and mid-size vessels that are loosely arranged together with an outer perimeter of dense connective tissue. The bundled fascicular structures collectively surrounded by this additional densely, more highly organized layer of fibrous tissue, houses the peripheral nerve contents and is known as the outer epineurium.
The outer epineurium connects the outer layer to the neighboring structures. A loose connective tissue fills the space between the nerve and the surrounding tissue in connection with the outer epineurium. There is thus an additional multi-layer boundary beyond the outer epineurium that runs along the entire trajectory of the nerve and is composed of an extraneural connective tissue known as the paraneurium. The paraneurium is a distinct multi-layer functional structure that enables the nerve to glide relative to other anatomic structures during muscular-skeletal movements.
To aid in locating a nerve branch, electrical stimulation was proposed in the year 1912. Electrical nerve stimulation was introduced from an understanding that nerve transmission is an electro-chemical response of excitation along the nerve (an axon). Introducing an electrical current stimulation to the body had the ability to elicit an indirect excitation of both the sensory and motor components of a nerve. This was found to provide a visual muscle contraction when the electrical stimulation was applied. Modulating the charge frequency and intensity lead to contraction and relaxation of muscle groups innervated by a nerve branch. This use of an indirect electrical charge to produce a nerve reaction to a specific nerve did not gain popularity because clinicians were unable to precisely control the various parameters of the current applied. The same deficiencies known when nerve stimulation was first introduced still exist today, including:                An inability to accurately modulate an applied electrical charge at a given distances to the surface of a nerve branch has made nerve stimulation limited in the identification of a specific nerve branch when using nerve stimulation as the primary means of nerve branch location. A variety of charge intensities are recommended at specific distances when approaching the nerve branch blindly ranging from an intensity of 2.0 mA to 0.2 mA. However, distance and intensity noted by a visual muscle twitch reaction does not correlate. Therefore, a reaction to a greater stimulation does not necessarily mean the needle is a greater distance to the intended nerve branch. And a reaction to a lower electric charge does not mean the needle is closer to the surface extraneural position and/or located within the nerve, i.e., achieved an intraneural location. In fact, there appears to be no consensus on the location of a needle (intraneural or extraneural) based on a reaction to an applied electrical charge irrespective of the intensity, frequency and duration applied to the nerve at a given distance.        A further deficiency of nerve stimulation technique is the inability to set the appropriate charge for a defined distance from the outer surface of the fascicle, i.e., Extra-Fascicular. It is more concerning if a high charge above 1.0 mA is utilized Intra-Fascicular, as it may cause a severe response by the patient or, even worse, result in irreversible damage from an excessive electrical charge applied directly on the axon. Hence there is an inability to determine what appropriate charge should be applied for a specific distance from the fascicle.        A further deficiency is that confounding variables make the use of nerve stimulation a non-specific technique. These are related to anatomic variations within a given patient as well as anatomic variation between different patients. The body is comprised of a variety of tissue types which include connective tissue of mineralized and non-mineralized tissues. These tissues are composed of water and collagen, adipose tissue (fat), muscle, fluids (blood), bone, cartilage, etc. Each of these tissues types provides a different resistance and/or capacitance to a charge when it is applied at a given distance to the intended target. The variables of tissue cannot be underestimated or anticipated. Hence current devices lack the ability to quantify a specific charge to a specific location. This has lead to an inability to produce predictable response to a given electrical charge when used as the primary means of determining location or proximity to a specific nerve.        
In summary, the variables of charge intensity, frequency and tissue resistance to the electric charge have made it difficult to standardize a technique to enable location of a specific nerve branch.